This invention relates to an arrangement for cooling a superconducting magnet winding, particularly the superconducting field winding in the rotor of an electric machine.
Cooling arrangements for the superconducting field winding in the rotor of an electric machine with at least one coolant chamber which, in the operating condition, contains a vaporous phase and a liquid phase of a coolant, which is fed into it via at least one feed line, a liquid space in the chamber occupied by the liquid phase being connected to coolant paths through the magnet winding and a vapor space in the chamber occupied by the gaseous phase having connected thereto at least one coolant discharge line with a predetermined high flow resistance, are known. For example, such an arrangement for cooling a superconducting magnet winding is described in the report of the Electric Power Research Institute, USA: "EPRI TD-255, Project 672-1, Final Report", August 1976, pages 45 to 52. This magnet winding is the superconducting field winding of a turbo-generator. The helium required for cooling this field winding is conducted from an external refrigeration plant, via a rotating helium coupler, centrally through the rotor shaft and is fed into a coolant chamber provided there near the axis. The coolant chamber contains a two phase mixture of liquid and gaseous coolant. Due to the centrifugal forces acting on this two phase mixture, the liquid phase is accumulated in regions away from the axis and the vaporous phase in regions of the coolant chamber near the axis. The liquid coolant flows through the coolant paths of the field winding, whereby the incident or produced heat is given off to the coolant. The coolant flow takes place, in this known cooling arrangement, due to a so-called self pumping effect, in thermosiphon loops. The heat absorbed by the coolant causes a temperature rise and partial evaporation. The coolant vapor then accumulates in a central subspace of the coolant chamber. From this vapor space, gaseous coolant is removed via a coolant discharge line and can further be used, for instance, for cooling parts of the rotor body.
Superconducting magnet windings must generally be designed so that they are able to withstand a sudden transition from the superconducting to the normally conducting state, called a quench, without damage. In the event of such a quench, part of the energy magnetically stored in the winding is converted into heat within a short time. The heat, which is partly given off to the coolant in the process, leads to evaporating of the coolant and thus to a pressure increase in the cooling arrangement.
This situation is particularly critical in turbogenerators with superconducting field windings. For, the magnetic energy stored in these windings is generally so large that in the event of a quench all the coolant evaporates. Since, as a rule, additional overpressure valves and rupture discs are to be avoided, the pressure being generated in the part of the cooling arrangement contained in the rotor can be reduced only via the coolant discharge lines of the rotor. However, the flow resistances of these discharge lines are relatively high due to the limited space because of the field current lines and the necessary evacuating lines that must be accommodated in a rotor. In larger generators, these discharge lines are furthermore relatively long, for instance, several meters. In addition, these lines can be made large enough only with difficulty, since otherwise undesirable secondary effects, such a secondary flow, can occur. Such effects lead to increased heat flow and thus to increased dissipation losses. The size of the coolant chamber in the center of the rotor body, which contains the coolant liquid and the coolant vapor, is likewise very limited. However, enough liquid must be stored in this chamber so that a sufficient coolant reserve is available for all operating conditions that can be expected. Thereby, only very little space is available for the coolant vapor in the center of the rotor. In addition, contrary to non-rotating windings, a considerably better heat transfer is effected in the rotating superconducting windings due to the large centrifugal forces. As a result, the coolant also evaporates faster than in comparable non-rotating magnet windings. In such cooling arrangements, the amount of coolant vapor can then increase very steeply in the event of a sudden quench, so that, for instance, a pressure of 50 bar or more is reached. Then, the danger of parts of the cooling arrangement being ruptured exists unless special measures are taken.
It is therefore desirable to provide, for the cooling arrangement of a magnet winding, sufficiently wide and short coolant discharge lines, so that the coolant vapor can be given off, for instance, via an overpressure valve, to the atmosphere or into a collecting tank. Such measures cannot be realized, however, in all cases due to space limitations.
The coolant feed canals of the cooling arrangement have therefore been arranged in a superconducting field winding in such a way that, in the event of a quench, a large part of the coolant is conducted out of the rotor in liquid form through these canals. In the cooling system provided in the rotor, only a correspondingly small amount of coolant can then evaporate (see Report of the Electric Power Research Institute, USA: "EPRI EL-577, Project 429-1, Final Report", November 1977, page 3-289). There have also been attempts to lay out the cooling arrangement in the rotor of a superconducting generator in such a manner that relatively little coolant at all is available in the rotor (Report of the Electric Power Research Institute, USA: "EPRI EL-663, vol. 1, Project 429-2, Final Report", March 1978, pages 2-112 to 2-114). Such a layout of the cooling arrangement is possible, however, only for field windings of relatively small machines.